Which IOS version do you have? The old 3750 IOS are quite buggy.So the CPU usage goes up to 70% momentarly when Nedi discovers the switch? I wouldn't worry too much about that, after your first post I thought that it is on 70% all time. Read the Cisco release notes for your IOS, often it is noted that SNMP can consume a lot of CPU. But at least with current IOS this doesn't degrade switch performance.

If you really worry about this, add a custom rate limiter for SNMP packets (but this will probably slow down Nedi discovery).

The problem is in order to read forwarding tables on Cisco switches NeDi needs to do something calle vlan community indexing. Effectively NeDi walks the forwarding mib with each vlan appended to the community (e.g. public@1, public@2 and so forth) in order to collect all nodes. You typically see a lot of warnings related to empty forwarding tables in this case as well.

Because of this I implemented the CLI alternative for getting forwarding tables. NeDi then uses telnet or SSH to get the info via CLI. As I just got a donation in regards to this topic I'm going to write some more documentation here:http://www.nedi.ch/doku.php?id=discovery:discovery (http://www.nedi.ch/doku.php?id=discovery:discovery)